Prior art printed circuit boards (PCBs) are formed using conductive metal interconnects (known as “traces”) formed on a dielectric substrate, where each surface carrying conductors is known as a “layer”. Each dielectric substrate has traces formed on one surface or on both surfaces to form a “sub”, which is one of the fundamental subassemblies of a multi-layer board, and by stacking several such dielectric cores having traces formed in them interspersed with bare dielectric layers, and laminating them together under temperature and pressure, a multi-layer printed circuit may be formed. The dielectric substrate may comprise an epoxy resin embedded in a fiber matrix such as glass fiber woven into a cloth. In one prior art fabrication method, copper is laminated onto the outer surfaces of a dielectric layer, the copper surfaces are patterned such as with a photoresist or photo sensitive film to create masked and unmasked regions, and then etched to form a conductive trace layer on one or both sides of the core dielectric. A stack of dielectric cores with conductive traces may then be laminated together to form a multi-layer PCB, and any required layer to layer interconnections made with vias, which are drilled holes plated with copper to form annular rings which provide connectivity from one trace layer to another.
One difficulty of prior art circuit board fabrication is that the addition of each new layer is a separate successive lamination step. For a four layer board (two core two-sided trace layers plus a pre-preg dielectric layer positioned in the middle), there is a single lamination step, and a 6 layer board is typically formed starting with the previously described 4 layer board after its lamination step, followed by sequentially laminating each additional two layer core to the previously laminated layers. Each lamination step requires several hours, pulling a vacuum, providing lamination pressure, and a heating and cooling cycle. In general, the number of lamination steps is equal to the number of subs less one, or for two layer subs and an n layer board (n>2) the number of lamination steps is
      n    2    -  1.After all of the lamination steps are completed, through holes are next drilled through the completed lamination at the via locations, and the via holes and any other through holes needing plating are desmeared and electroless plated through to provide electrical connectivity from one layer to another. For high speed signals, the plated through holes which transit the entire thickness of the finished pcb may result in transmission line reflections and impedance discontinuities at the through-hole structures.
Another problem of fabricating multi-layer boards is that the traces on the surfaces of each layer may tend to migrate laterally during the application of lamination heat and mechanical pressure, since the traces are typically positioned above the surface of the PCB, and they are typically exposed after etching away an original copper foil from the surface of the PCB.
It is desired to provide an alternative lamination method which provides for a single lamination step, which prevents the lateral migration of the traces during lamination, and which also provides electrical connectivity between layers such that the step of drilling through the laminate of the prior art is eliminated.